Special Feature SECTION
WATERFALLS … RESCUES AND RECOVERIES Bob Twomey
I
t is that time of year when people will inevitably visit these powerful natural features, to awe at their beauty, to photograph them … and for some, to be hurt or killed by their unsuspected dangers. It is only May as I write this article and already there have been waterfalls accidents in western North Carolina. Each waterfall is different. Some are on small streams, others on various sizes of rivers. Some have shallow, broad flows (see Figure 1). Others have concentrated flows over smaller surface areas, which can often lead to accelerated flows of great force, as determined by cubic feet per second (CFS) flow rates (see Figure 2). Similarly, some waterfalls have what is
formation, some waterfalls have vertical drops to pools below (see Figure 3). They may have a substantial drop or they may have a drop of a few feet but have in common no contact with the rock surface during the “free-fall.” Each is different and each has its hydraulic characteristics. So why would someone in rescue think to analyze a waterfall? After all, a waterfall is a waterfall, right? Well, yes and no. Over 48 years of working with rivers and streams in my former career with the Natural Resources Conservation Service and rescue, I have learned a few things about water flows, rivers … and rescues therein. Sometimes the things I will share with you here, at least in waterfall rescues and recoveries, were learned the hard
Figure 1: River flow spread out over large rock surface area
Figure 2: More concentrated flows over less rock surface area
known as “braided flows,” as in the photo showing concentrated flow, whereby the channel water volume is split into multiple channels. This is often determined by water volume and rock shape or irregularities in the rock surface. Concentrated flows are generally deeper and the water velocity is often faster. Based upon rock
channel is measured in CFS (see Figure 4). One cubic foot of water contains 7.48 gallons; therefore, a creek with a 1 CFS flow is flowing at 7.48 gallons per second. A flow of 10 CFS would be 74.8 gallons per second, and so on. However, a river channel is not perfectly dimensioned or said another way, the channel is not square with 90-degree sides and bottom like a concrete drainage canal would be. Thus, even though the flow rate past a certain point might be 74.8 gallons per second, parts of the channel flow are deeper than other parts due to the irregularly shaped channel. This deepest part of the cross-section of a channel is called the thalweg. Friction between the flowing water and the rock surface and edges of the channel will cause
Figure 4: Many cubic feet per minute flowing over this waterfall
Figure 3: Vertical drop waterfall to pool below
way. Experience can be a stern teacher about things in nature; a determined rescuer will learn from course work, training, and then practical evolutions and real rescues....so that each subsequent rescue in a waterfall setting will be safer and hopefully easier. First is river flow. The volume of water flowing down any given
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Also, rescuers MUST always keep this concept in mind: for every 1 cubic foot of water displaced by the body as one proceeds into deeper water, 62.4 lbs. are subtracted from the rescuer’s body weight, since 1 cubic foot of water equals 62.4 lbs. So now, a 200 lb. rescuer displacing 1 cubic foot of water now only weighs 137.6 lbs. The force of the moving water will ALWAYS move a lighter object in the flow easier than a heavier object. The lesson here? NEVER go into moving water without a secure rope anchor system attached to you if you’re in the water current on any waterfall. So now you have the concepts and interrelationships of water volume, flow velocity, and displacement by your body in water. The deeper you proceed into
slower flow and with less force than the flow in deeper water. The thalweg, being the deeper part of the channel cross-section, therefore, has more force, or more powerful flow, than the shallower sections. This area is particularly dangerous to rescuers due to the power exerted upon the rescuer within this section of the channel.
the flow, the less you weigh and the more force there is exerted upon your body, (which is now lighter than your weight on dry land), and the faster the flow (see Figure 5). In this photo example, the victim or rescuer will still fall to the bottom of the falls once you start sliding. But IF you must move down this waterfall, in a very careful rappel in the water, the obvious choice is in the foreground where the water is only inches deep rather than in the thalweg, which is several feet deep and flows many linear feet per second. And yes, over the years, we have had to rappel down the face of waterfalls to reach victims snagged on trees or rocks in the waterfall itself. Some waterfalls have what is known as a “braided flow”, that is, split flow channels around rocks or vegetation within the “channel.” (see Figure 6). These are falls where rappels through the water and on wet rocks will be needed. Next is the “biological component” of waterfalls. Algae growth on rock surfaces is very common, even on rock surfaces that WWW.CAROLINAFIREJOURNAL.COM
